1. Field of the Invention
This invention relates to the field of purging tanks and vessels and more particularly, to methods and apparatus for purging volatile compounds from tanks and vessels in preparation for maintenance and cleaning.
2. Description of the Related Art
Volatile liquids, such as benzene, petroleum and the like, are often stored in tanks at bulk terminals, refineries and end-user facilities, and transported in tanks aboard barges or ships, tank trucks and rail cars. All such containers shall be referred to herein as liquid storage vessels. While resident in these liquid storage vessels, volatilization of the liquid occurs leaving residual vapors which must be removed before workmen can be permitted to enter the vessel and before the vessel can be filled with a different liquid.
In some cases, such residual vapors are purged by flooding liquid storage vessels with a sufficient volume of water or air to entrain the vapors and carry them out of the vessel. The resulting mixture of diluted vapors, in many cases, is simply emitted to the atmosphere and surrounding water supply where they pollute the environment. Emissions handled in this manner lead to severe environmental hazards. For example, hydrocarbons are a major contributor to the formation of smog, which has been proven to increase respiratory disorders among the population.
In addition to these environmental problems, water-flushing facilities must overcome many economic hurdles. Adequate water for such facilities may be expensive due to limited water resources or to restrictions concerning the reuse or recycling of the water. If the water must be reused or recycled, it must be treated to remove contaminants that might pollute the environment or contaminate the next vessel to be flushed.
There have been several patents in the prior art that attempted to address the problem of removing vapors from storage tanks and collecting the gases that are forced out of the storage tank to reuse such gases for combustion or in other vapor-handling units. For example, U.S. Pat. No. 5,476,126 shows apparatus and methods for removing vapors from a storage tank by introducing a purge medium, such as CO2, into the storage vessel and establishing a uniform and continuous stratified interface between the purge medium and the volatile organic compound layer. As the purge medium is continuously fed into the storage tank, the continuous stratified interface moves within the vessel, thereby purging the undiluted volatile vapors from the vessel into the vapor-handling unit where the vapors may, for example, be combusted, adsorbed onto an adsorption bed and/or condensed or cooled to a liquid form by a refrigeration system.
U.S. Pat. No. 1,918,100 shows a gas-gathering system which is basically a closed system in which vapors which collect in a storage tank are pumped into a secondary vapor storage tank partially filled with water and from the vapor storage tank are recaptured through a compression and condensing process to provide dry gas for other uses such as combustion. The patent states as its primary objective the provision of a method and apparatus for maintaining a hydrocarbon gas at all times within the storage tanks above the liquid levels thereof with the specific end in view of preventing air or oxygen from entering the tanks and mixing with the gases contained therein.
Regardless of the progress that has been made in the area of minimizing release of volatile materials into the atmosphere during the preparation of storage vessels for maintenance, there are still areas that need to be improved. It is generally preferred to recover the volatile materials as a liquid, rather than combusting the materials or adsorbing the materials onto an adsorption bed. What is needed are apparatus and methods for recovering the volatile materials that are removed from a storage vessel so that the material may utilized as a product or raw material rather than being disposed of.